1. Field of the Invention
The present invention relates to a method of gluing together first and second disc elements to form a composite disc wherein a quantity of glue is applied on the first disc element in the form of a ring, the ring being disposed concentrically with a center hole through the first disc element, and wherein a second disc element, which also has a center hole and which is parallel with the first disc element, is moved in parallel with the glue ring during coaxial alignment between the center holes of respective disc elements, such as to enable the glue to be spread between the surfaces of the disc elements as a result of pressure and capillary action.
The present invention relates to a method of gluing together first and second disc elements to form a composite disc of the kind defined in the preamble of claim 1.
2. Description to the Related Art
There are, at present, four different types of DVDs, these being designated DVD5, DVD9, DVD10 and DVD18. All four formats consist of two 0.6 mm disc elements, so-called substrates, which are bonded together with a so-called bonding material, which is most often a transparent UV-cured glue. The bonding material used in the formats DVD9 and DVD18 must be transparent.
The most usual method of bonding the substrates together is to place the bottom substrate on a rotatable table and applying glue to the substrate. A linear flow piston pump delivers a predetermined amount of glue, about 0.5 ml, through a cannula whose tip is placed on a radius of about 30 mm from the centre of the substrate. The substrate is rotated through one or more turns while delivering glue to the substrate at the same time. There is thus provided on the bottom substrate a ring of glue that has an outer diameter of about 60 mm.
The top substrate is then pressed slowly down onto the glue ring, causing the glue to spread between the substrates partly as a result of the pressure exerted and partly through capillary action.
The known technique used in practice thus involves applying a predetermined amount of glue onto a first disc element in the form of a concentric ring, and then displacing a second disc element arranged plane-parallel with the first disc element in parallel with said element so that the glue will spread partly as a result of the pressure applied and partly as a result of capillary action. The glue is preferably applied whilst the first disc element rotates, and the first disc element is preferably orientated horizontally.
The disc elements/the substrates comprise plastic discs that have an information carrying layer, for instance an aluminium layer.
The aforedescribed gluing method is encumbered with serious drawbacks, however, due to the formation of air bubbles in the glue as the top substrate is pressed into contact with the glue on the bottom substrate. This formation of air bubbles is caused by a combination of the surface tension of the glue, the capillary action, and the static electric charge on the substrates, therewith causing uncontrolled jumping of the glue to the top substrate as the substrates come into contact with each other, which results in a foam formation. When the substrates are highly charged, glue particles are liable to jump to the top substrate before contact with the glue has been made.
Although these bubbles have not the same significance in respect of the formats DVD5 and DVD10, they can cause oxidation of the aluminium layer on the disc in certain cases, resulting in disturbances in operation.
On the other hand, it is required that no air bubbles are formed in the glue when producing the formats DVD9 and DVD18, since in the case of these formats the laser reads through the glue coating; even microbubbles will create reading interference.
The object of the present invention is to endeavor to eliminate the aforesaid drawbacks and to provide a bonding method that will provide a uniform glue film and a glue film that extends right up to the center hole of the substrates and which completely lacks air bubbles.
This object is achieved with the present invention in which, when bonding a first disc element to a second disc element with the aid of glue to form a composite disc, a center peg is sealingly inserted in the center hole of the first disc element, and a glue ring is applied in contact with the peripheral surface of the peg.
An important feature of the invention resides in placing a peg in the centre hole of the first disc element, in sealing contact with the edge of the hole. The glue ring is applied suitably in contact with the peg, so as to enable the greatest thickness of the glue ring to be established in the close proximity of the surface of the peg.
The glue and/or the peg is/are suitably chosen so that the glue can be drawn up along the vertically orientated peg through the medium of surface tension and so that the glue ring will obtain a cross-sectional shape that is generally similar to that of a right angled triangle. The glue ring applied to the first disc element is suitably formed by a predetermined amount of glue, which is distributed uniformly around the disc periphery. After the glue has been spread out, the glue can be cured suitably in the region nearest the centre hole of the disc.
The glue quantity may be applied in the form of a constant glue flow which is deposited around a predetermined circular path that is concentrical to the first disc element; the glue flow is chosen so that the glue will be applied around the first disc element in a full revolution of the disc element or in a number of full revolutions.
A delimiting ring having a circular inner surface may be placed temporarily on the first disc element, the inner surface being coaxial with the disc element, and therewith defining a radially outer spread limit with respect to the glue ring prior to applying the second disc element.
The second disc element may be supported on a horizontal surface of a table that is rotated about a vertical axis. Additionally, the second disc element may be pressed against the glue ring, optionally with the aid of a cylindrical press foot that is applied coaxially to the second disc element.
Furthermore, the second disc element may be first moved toward the first disc element at a relatively high speed until the disc reaches contact with the glue ring; movement of the second disc element toward the first disc element may then be continued, but at a relatively low speed. The first and second disc elements may also be rotated commonly about their respective coaxial axes so as to spread the glue layer radially.
The present invention further includes a method of forming a composite disc from first and second disc elements using a peg in a center hole of the first disc element, characterized by choosing a glue that can be cured by radiation, curing the glue layer spread out between the disc elements by irradiating the layer in the region nearest the peg, and thereafter moving the mutually bonded disc elements with cured glue in the region nearest the center holes of the disc elements to a curing apparatus in which the pair of mutually bonded elements are rotated while being irradiated to finally cure the glue layer.
The present invention further includes a method of forming a composite disc from first and second disc elements using a peg in a center hole of the first disc element, in which the glue ring is applied by delivering to the first disc element a predetermined quantity of glue with the aid of a piston pump set to a given glue flow, via an outlet passageway which opens towards the peg through a stationary outlet opening, while rotating the first disc element at a speed which is synchronized to the pump flow so that the glue string will be deposited through one or more complete revolutions of the disc element and form a glue ring with the glue distributed uniformly around the perimeter of the element.
The present invention may further be characterized by choosing the exposed material on the peripheral surface of the peg so that the glue ring will have its greatest thickness in the longitudinal direction of the peg and adjacent the perimeter surface of the peg. In addition, at least a part of the outer surface of the peg may be comprised of a springy, pliant material so as to seal against the edges of the center hole, the outer surface of the peg being adapted to sealingly bridge the edges of the center holes.